Why not viatu vikijivu or something like that??? Why ekundu becomes mwekundu, but not this one?
Grey is rangi ya kijivu, the colour of ash. Ash is jivu, so the ki- is already something that has been added on. Haven't figured out why yet, but green, kijani (rangi ya kijani, the colour of leaves), is another.
I'm working on a theory, but if anyone who knows turns up in the meantime, hopefully they can elaborate.
Edit to add: I'm being stupid. "Colour of X" formations, X is a noun, not an adjective, so it won't take agreement from another noun.
Yeah, basically -eusi, -eupe and -ekundu are the only colour adjectives. The rest are all nouns.